This invention is directed to an electron beam microfabrication apparatus and method wherein a curved photocathode is part of a demagnifying electron optical system. When the photocathode is excited through a mask pattern the resultant electrons are demagnified and focused onto a wafer.
The prior art includes electron projection from a photocathode. A typical cathode projection microfabrication system reproduces patterns with about a 0.6 .mu.m line width at 1:1 magnification onto a silicon wafer. The required pattern is defined in a mask on a flat quartz substrate and a photocathode is evaporated on top. The mask is illuminated from behind with ultraviolet light so that electrons are emitted from the clear mask areas but not from the dark areas. The photoelectrons are accelerated by a high uniform electric field onto the silicon slice which is held opposite. The photoelectrons are focussed by a uniform magnetic field parallel to the axis of the mask and wafer. This electron imaging technique is disclosed in W. R. Livesay, "Integrated Circuit Production with Electron Beams," J. Vac. Sci. Technol., 10, 1028-32 (Nov./Dec. 1973); J. E. Piquendar, "Nanoelectronics," Proc. Fifth Int'l Conf. on Electron and Ion Beam Science and Technology, (May 1972, Houston) Electrochemical Society, Princeton, N.J., 1972, p. 31; and J. P. Scott, "An Electron Image Projector with Automatic Alignment," IEEE Trans. on Electron Devices, ED-22, 409-13 (July 1975).
Unfortunately the practical embodiment of such a system poses problems for submicron lithography. The problems which arise include the fact that the pattern printed on the photomask must exceed the accuracy of the pattern which will be produced on the wafer, to ensure reproducibility. Another problem is the distortion of the accelerating electric field which is produced by the wafer holder, and this distortion distorts the projection. Furthermore, electrostatic charging of dust particles on the wafer, and charging of the resist both introduce factors which result in non-repeatable image distortion. Wafer bowing is another effect which produces pattern distortion. The deviation from flatness of the photocathode produces pattern distortion similar to wafer bowing. However, since the electrons move very slowly close to the cathode the tolerances on the emitter flatness must be much tighter.
Another problem with the parallel projection system is that the thermal effects of parallel exposure produce a temperature rise of about 6.degree. C. in the wafer with resultant bowing which will lead to blurring of the image point. Another problem of this projection system is that backscattered electrons cannot escape. Electrons which are backscattered from the wafer are emitted into an electric field which drives them back into the semiconductor wafer, thus causing electron impingement where is it not desired. In addition, the electron optics of the parallel projection system do not permit the introduction of an aperture into the system, and hence the resolution is determined by velocity spread of the emitted photoelectrons.
In order to overcome many of the above limitations a ten-to-one demagnifying electron projection system has been built. That system is disclosed in M. B. Heritage, "Electron-Projection Microfabrication System," J. Vac. Sci. Technol., 12, 1135-40 (Nov./Dec. 1975). The mask is a self supporting stencil with a desired pattern cut out so that a flood electron beam may be transmitted through it. The exposed field size is 3 mm.times.3 mm and the mask has an area of 30 mm.times.30 mm. The minimum linewidth is 0.25 micron and is defined to 0.05 micron. The exposure time for PMMA resist (the conventional modern photoresist material employed in this service) is about 0.1 second for each 3 mm square field.
The stencil is in field free space at final voltage and acts to produce the electron distribution by stopping unwanted electrons.
The problems with the above-described demagnification projection system include the fact that it is an extremely large system and it is therefore subject to perturbation by stray fields. The system is complex and therefore expensive. The self-supporting stencil mask is difficult to manufacture and may pose power dissipation problems. Furthermore, the field coverage is limited to about 3 mm.times.3 mm by off axis aberrations. That prior art system includes an electron gun which supplies electrons through a series of electro-optical lenses to provide a broad (30 mm square) flood electron beam with parallel paths. The self-supporting mask is placed in this beam so that some of the electrons are intercepted in accordance with the mask pattern. The electrons which pass are focussed by a first projector lens through an aperture to a second projector lens which directs the patterned electron beam again into a parallel beam with a maximum 3 mm square size onto the wafer. That system is about 152 centimeters long, from the electron gun to the wafer and is positoned in a 40 centimeter diameter tube. This large size makes it subject to perturbations from external fields.
Thus there is need for an electron beam microfabrication apparatus which is an improvement over these prior art devices.